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air seal attic: plywood ok?

Hi,
In the name making my HVAC more effective, may i ask a question about plywood in the attic. The house has a 10x6 x4 foot deep sized built in wooden display case, with an area for a minifridge. To spare you details, the built is connected to openings to the attic with no insulation. It causes a great deal of "ventilation" to leak all around and between the entire built in. The opening from the display case, to the attic is irregular--parts of rafters are in the way. Its not like i can easily just throw a roll of insulation over the area. I would need cut plywood into strips and squares, nail them in place, then lay insulation over it. Can i do this with plywood, or do i have to use drywall which i am less familiar with? I'm specifically worried about risk of fire? There is a recessed light in the built in, but the plywood i would overlay would be about 2 feet above it/then insulation.

I am a white collar type guy, and I would hire someone to do the job, but i'm not sure who would do it right---I have a feeling by the time i spent talking and interviewing people, it would be easier to do myself.

what you are describing is called a thermal bypass.
in the attic you can see the top of the 'built in' at a lower
level than the rest of the attic floor.

I use ductboard to seal between ceiling joists.
measure & cut to fit area to be filled. cut long enough to cover opening
and overlap onto attic floor. cut it to fit tightly between joists.
caulk ductboard to attic floor, secure with button cap nails.
caulk to joists to make air tight.
once the opening is closed & sealed then put batt insulation
on top of ductboard.

over the years I've tried many methods. plywood & sheetrock
were difficult to install, because sometimes you need to cut to
fit as you put the plywood or sheetrock in place. ductboard
is much easier to work in place.

from inside the house you can do additional air sealing by
using clear caulk to seal inside the built in.

with the recessed light in the ceiling of the built in and the
ductboard at attic floor there is no worry about overheating
of fixture. 2' is plenty of space.

it isn't uncommon to see batts covering openings like this.
I've learned to move batts around to look for these thermal
bypasses. fireplace inserts, dropped ceilings at showers,
closets and like you have...built ins.

have to agree..by the time you find someone to do the work
you could have done it yourself. thats how I wind up doing
these types of things!

best of luck.

The cure of the part should not be attempted without the cure of the whole. ~Plato

I was a little hesitant of plywood, thinking that drywall would be more of a fire stop, but at least the ductboard can't burn. The area is tight to work in, so the ductboard seems much more doable.

Be sure you catch the part where energy rater suggested working from the attic vs. working from below (if I interpreted what you described in the OP correctly). Your challenge as a homeowner may be finding duct board...Home Depot used to carry it but I'm not sure they still do. It is sold in HVAC supply houses, many of them may not sell direct to the public since they are wholesale. You do want duct board vs. foam sheathing, as most foam sheathing requires some form of fire resistant barrier between it and the rest of the building.

Thanks for the help. As Shophound suggested, I cannot easily find duct board.

Back to plywood---is there a code (or more importantly safety) issue with plywood covering this sort of thing, then insulation? I know drywall has flame resistant properties but i'm not familiar with working with it.

if your local HD doesn't have it in stock...they can order it for you.
much much easier to work with compared to sheetrock.

to work with sheetrock, you use a straight edge & a razor knife.
first you score the sheetrock on both sides, then chose a side
to make a deep cut.
turn the sheetrock on its side & crack along
cut. scoring the sheetrock keeps paper backing from tearing.

you may have to practice a bit first..but sheetrock is cheaper
by the sheet & you'll have excess.

caulk under sheetrock to attic floor when ready to install piece.
you can't caulk edges because sheetrock dust keeps it from
making a seal.

best of luck.

The cure of the part should not be attempted without the cure of the whole. ~Plato